The invention relates to a prism assembly, and in particular to a color separation prism assembly for a projector.
FIG. 1 is a schematic view of an optical system disclosed in U.S. Pat. No. 5,621,486, with a Philips prism assembly 100 separating incident light into colors, such as red (R), blue (B), and green (G). As shown in FIG. 1, the Philips prism assembly 100 comprises three prisms 102, 104, and 106, and two color separating surfaces 108 and 110 with dichroic coatings. Air gap 114 is formed between the prisms 102 and 104. When incident light I enters the prism assembly 100, the color separating surface 110 reflects red light IR and allows blue light IB and green light IG to penetrate through. The reflected red light IR is further reflected by an inner surface of the prism 102 to enter the liquid crystal light valve (LCLV) 112R. After incident light I with filtered red light IR passes through the air gap 114, it impinges upon the color separating surface 108, filtering the reflected blue light IB and permitting penetration of the green light IG. Thus, the green light IG directly penetrates the color separating surface 108 and enters the LCLV 112G. After the blue light IB is reflected by the color separating surface 108, total reflection occurs at the interface between the air gap 114 and the prism 104 such that the blue light IB is reflected and enters the LCLV 112B.
The disclosed method reduces volume of the prism assembly, and the light beam can impinge upon the color separating surface at a smaller incident angle; however, before separating the blue light IB and the green light IG, the light must penetrate through the air gap 114 in an inclined direction. The light paths at different angles will reduce image accuracy. Furthermore, the back focal length is longer.
FIG. 2 is a schematic view of a cross dichroic prism of a color projector 200 separating incident light into red, blue, and green colors, as disclosed in U.S. Pat. No. 5,153,752. As shown in FIG. 2, an S polarized component and a P polarized component of the incident light I of the color projector 200 are reflected by polarized beam splitters 202a and 202b, respectively, and enter the cross dichroic prism 204. The cross dichroic prism 204 is constructed by a plurality of cross dichroic mirrors 204a, 204b, 204c, and 204d, as shown by diagonal lines of a quadrangular cross section. The dichroic mirrors 204a and 204d have characteristics to filter and reflect only the blue light component. The dichroic mirrors 204b and 204c have characteristics to filter and reflect only the red light component such that the red light (RS), green light (GS), and blue light (BS) are directed to corresponding liquid crystal light valves 208R, 208G, and 208B, respectively. The liquid crystal light valves can modulate and reflect individual color light components, such that the color light components are reflected and combined to enter the projection lens system 206.
Although the color projector 200 shortens the back focal length to avoid the disadvantages of the previously mentioned prism assembly 100, the designed light beam impinges on the color separating surfaces at an extremely large incident angle of 45°. If the incident angle at the color separating surfaces is too large, the polarizing light spectrum through the dichroic mirrors is sensitively shifted with variation of incident angle, reducing color purity after color separation. The overall color light separation efficiency of the prism assembly is reduced, accordingly. Furthermore, during manufacture of the cross dichroic prism 204, it is difficult to manufacture the dichroic mirrors in alternative arrangements.